Photoluminescence in ZnO:Co2+ (0.01%-5%) Nanoparticles, Nanowires, Thin Films, and Single Crystals as a Function of Pressure and Temperature: Exploring Electron-Phonon Interactions

This work investigates the electronic structure and photoluminescence properties of Co2+-doped ZnO and their pressure and temperature dependences through high-resolution absorption and emission spectroscopy as a function of Co2+ concentration and their structural conformations as a single crystal, thin film, nanowire, and nanoparticle. Absorption and emission spectra of diluted ZnO:Co2+ (0.01 mol %) can be related to the T-4(1)(P) -> (4)A(2)(F) transition of CoO4 (T-d), contrary to MgAl2O4:Co2+ and ZnAl2O4:Co2+ spinels in which the red emission is ascribed to the E-2(G) -> (4)A(2)(F) transition. We show that the low-temperature emission band consists of a T-4(1) (P) zero-phonon line and a phonon-sideband, which is described in terms of the phonon density of states within an intermediate coupling scheme (S = 1.35) involving all ZnO lattice phonons. Increasing pressure to the sample shifts the zero-phonon line to higher energy as expected for the T-4(1)(P) state upon compression. The low-temperature emission quenches above 5 GPa as a consequence of the pressure-induced wurtzite to rock-salt structural phase transition, yielding a change of Co2+ coordination from 4-fold T-d to 6-fold O-h. We also show that the optical properties of ZnO:Co2+ (T-d) are similar, independent of the structural conformation of the host and the cobalt concentration. The Co2+ enters into regular Zn2+ sites in low concentration systems (less than 5% of Co2+), although some slight shifts and peak broadening appear as the dimensionality of the sample decreases. These structural effects on the optical spectra are also Supported by Raman spectroscopy.